As one skilled in the gas turbine engine technology knows, there has been and continues to be a desire to reduce the clearance between the tips of the blades of the engine's rotors and the surrounding shrouds, sometimes referred to as blade outer air seal (BOAS) or blade segments, so as to improve the operating efficiency of the engine. Scientists and engineers have developed a number of systems for reducing the clearance with the aim of making it as tight as possible and a continuing effort is currently being made to make these clearances even tighter without adversely affecting the structural integrity of the component parts. Generically speaking, there are two types of systems that have played a prominent role in this area of technology, namely, the active clearance control and the passive clearance control.
The active clearance control typically requires a valve, plumbing, a control and actuator for positioning the valve at given times during the operating envelope of the engine. In particular, where cool air is utilized as the medium for cooling the components, cool air is circulated in the vicinity of the operating parts, which is generally during the cruise condition of the aircraft being powered by the engine. The clearance between the rotor and tips of the blade is designed so that the tips of the blades do not rub up against the blade segments during the high powered operation of the engine, such as, during take-off when the blade disc and blades expand as a result of the high temperature and centrifugal loads. As soon as the engine power is lowered to the cruise condition, and the component parts contract, the active clearance control is actuated to deliver cooler air to the adjacent components to reduce the heat load of the case or adjacent components. The result is to shrink the case and those adjacent component and position them closer to the tips of the blades so as to minimize the leakage of engine working medium between the tips of the blades of the rotor and the blade segments. Obviously, the leakage represents loss of energy that has been added to the engine working medium and represents a deficit in engine efficiency. Of course, the down side of the active clearance control is that it adds complexity to engine, requires valving and moving parts, a control system and in many installations it requires piping that surrounds the engine case to shower the case with cooling air to effectuate the contraction thereof.
An example of an active clearance control is described in U.S. Pat. No. 4,069,662 granted on Jan. 24, 1978 to Redinger, Jr. et al and entitled CLEARANCE CONTROL FOR A GAS TURBINE ENGINE. As noted above, this control impinges cool air from pipes surrounding the engine case onto the engine case to cause the case to shrink and position the blade segments closer to the tips of the turbine blades in order to reduce clearance during the cruise operation of the aircraft.
The passive clearance control also serves to maintain low clearance between the tips of the blades of the rotor and the blade segments and performs this function without the use of valves, controls and typically without extra piping. For example, when applied to internally cooled turbine blades, the spent cooling air discharging from the turbine is oriented toward the pressure side of the turbine blade and adjacent to the tip of the turbine blade in order to define a fluid dam in the gap so as to seal off the gap entrance and hence, reduce the leakage of the engine working medium. Such a system is described in U.S. Pat. No. 5,282,721 granted on Feb. 1, 1994 to Robert J. Kildea and entitled PASSIVE CLEARANCE SYSTEM FOR TURBINE BLADES.
Other attempts to reduce clearances include the use materials that exhibit the desired coefficient of expansion and/or heat transfer coefficients.
One of the major obstacles that the designer needs to overcome is the selection of a system that will contemplate the relatively faster expansion/contraction of the lower mass blades and the slower expansion/contraction of the disc supporting the blades. Notwithstanding the advances made in this technology, all of the systems alluded to in the above paragraphs do not achieve a relatively constant clearance during the transient operation of the engine. In other words, since the blades and discs have different coefficients of expansion as do the surrounding shrouds and blade segments, the gap dimensions vary during these transients. Hence, if the gap increased during the transient, there will be an increase in leakage of the engine working medium flowing through this gap.
This invention obviates this problem noted in the immediate above paragraph by providing specific material of the component parts and designing the blade segment assembly so that the blade segments will effectively follow the slope of the growth and the shrinkage of the rotor and blade. Accordingly, the expansion/contraction slope of the blade segments match the expansion/contraction slope of the blades and disc of the rotor for all conditions of the engine's operating envelope. This invention contemplates the use of the low thermal expansion material, such as Nilo-K or Inco 909 for the support ring and it, together with the design of the retainer and its relationship to the blade segments permit the floating of the blade segments which effectuates a match of the slopes of the rotating and non-rotating parts.